In an infrared (IR) transmitting system, the IR radiation source is the major power consuming component of the system. If the IR source can be designed to more efficiently convert electrical input power into radiation of the desired wavelengths, the efficiency of the system will be improved. In airborne equipment especially, size, weight and operating efficiency are basic design considerations.
Unlike most other materials, boron nitride (BN) has a spectral emittance particularly suited for efficient production of IR radiation at wavelengths longer than approximately 3 micrometers. Additionally, boron nitride is a highly refractory dielectric material. It has excellent thermal shock resistance and is chemically compatible with many refractory materials at high temperatures. Boron nitride is also considerably less brittle than most ceramic materials, suggesting long-lived performance in high vibration environments. The high temperature usefulness of pure boron nitride is limited mainly by its tendency, when heated, to disassociate to elemental boron and nitrogen. Boron nitride also reacts with the air, generating boric oxide (B.sub.2 O.sub.3) when heated over 1000.degree. C.
The efficiency of boron nitride results from its low emissivity (therefore low radiance) at short wave lengths (visible to 3 micrometers) and an emissivity approaching unity (therefore high radiance) between 3.5 and 5.7 micrometers. Consequently, boron nitride requires less input power does a blackbody source to provide equal radiated power in the 3-5 micrometer band.
In the prior art, pulsed Cesium Arc lamps have been used as IR sources, but such lamps have had marginal efficiency in the wavelength band of interest (3-5 micrometers). Further, such pulsed lamps are short-lived. Mechanically modulated incandescent lamps in general have a better lifetime potential and are more efficient in the 3-5 micrometer band than arc lamps. This is because arc lamps must operate at a higher temperature than incandescent lamps. Thus, since the maximum spectral radiance of a body shifts towards shorter wavelengths as its temperature is increased, the incandescent lamp has a higher fractional output in the spectral band of interest. However, a disadvantage of most incandescent radiators is that they act as blackbody or graybody sources, thereby wasting power by emitting significant amounts of nonuseful visible and short wavelength IR radiation. Further, prior art selectivity radiating IR sources, such as heated pyrex glass sources, are not bright enough, due to the inherent temperature limitations involved with such materials characterized by low refractoriness.
Although boron nitride has been used as a radiator in the prior art in combination with a heater filament, the uses have been as a source for visible radiation, not selective IR radiation. U.S. Pat. No. 2,164,183 to Van Liempt, et al discloses the use of boron nitride as a radiator. The advantage taught in the Van Liempt patent is the use of boron nitride rather than a conventional tungsten filament in the generation of visible radiation. The use of boron nitride apparently enabled uniform filament evaporation, thus lengthening the life of the lamp.
U.S. Pat. No. 1,385,608 to Darrah also described the use of boron nitride to generate visible radiation rather than as a selective radiative IR source. The Darrah apparatus employed a boron nitride envelope as an insulator for enveloping the heating element, enabling the heating wire to exist as a temperature very near to or even exceeding its normal melting point. The Darrah specification does describe the concept of generating selective radiation, but suggests that this is enabled only when the refractory insulator, e.g. BN, is coated with various compounds such as metallic tungsten, which themselves provide the characteristic of selective radiation. The selective radiation characteristics of boron nitride, however, were not taught.
Further, Darrah failed to discover the chemical incompatibility of boron nitride and tungsten at high temperatures (approximately 1500.degree. C. and above). Sealed lamps recently constructed therewith exploded due to the generation of nitrogen gas and tungsten boride. Thus, if tungsten is used, it must not be constructed to be touching or otherwise in a reactive relationship with the boron nitride.